1. Technical Field
The present invention relates to image processing capable of preventing so-called “banding”.
2. Related Art
An image forming device having an ink ejecting mechanism, such as an ink jet printer, has a plurality of nozzles for ejecting an ink dot in a droplet form, so as to perform high-speed printing. The nozzles are mounted on a printing head. An ink jet printer causes the printing heads to eject ink dots by moving a sheet of paper (or a print media) in a sheet feeding direction and moving the printing heads repeatedly in a direction orthogonal to the sheet feeding direction, thereby forms images on the paper. This kind of printer is referred to as a “multi-pass printer”. The printing speed of a multi-pass printer is restricted since the multi-pass printer needs a two-directional scan. Therefore, an improvement in printing speed is required.
Reducing the scan dimension into a one-dimensional scan, for example, scanning only in a sheet feeding direction, referred to as single-pass printing, is one of the approaches to improve printing speed. It is necessary for single-pass printing to extend the size of a printing head to a width greater than the width of the printing paper. This kind of printer, in other words, a printer having a printing head which is larger than the width of printing paper, is referred to as a “line head printer”. In a printing head of a line head printer, it is necessary to layout nozzles at a regular distance from each other on the printing head. However, it is difficult to fabricate a printing head in which the nozzles are laid out at exact intervals, because of fabrication errors. The dispersion of a distance between two adjacent nozzles causes the actual position of an ink dot formed on printing paper to be different from an ideal position. Furthermore, a skew of a nozzle also causes the actual position of an ink dot formed on printing paper to be different from an ideal position. The phenomenon of causing a difference between the ideal position and the actual position, is called “splash bending”. In a single-pass printer such as a line head printer, the phenomenon of splash bending causes white bands or dark bands to appear on the print product. The white band appears where a distance between two adjacent nozzles is greater than the ideal distance, and the dark band appears where a distance between two adjacent nozzles is less than the ideal distance. This phenomenon is referred to as “banding”.
JP-T-2004-531982 discloses an approach to prevent banding. JP-T-2004-531982 discloses binarization using a dither matrix that has characteristics of both so-called “clustered dither matrix” and “dispersed dither matrix”. According to JP-T-2004-531982, dot seeds are randomly laid-out in a dither matrix and dot clusters grow around the dot seeds.